The invention is particularly suitable for checking mechanical pieces with extremely small dimensions, like component parts of hard disk storage units. Such units include magnetic read/write heads—for reading/writing information on disks—coupled to extremely small-dimensioned sliders, for example in the order of the millimeter or even less. Stationary supports carry, by means of an elastic coupling, the sliders that make the magnetic head “float” over the surface of the associated disk when the latter is driven into rotation, for example by an electric motor. The rotation of the disks and the specific shape of the sliders generate a lift that tends to displace the sliders and the magnetic heads away from the surfaces of the associated disks, opposing the force applied by the elastic coupling. The balancing of the elastic coupling force and of the lift allows the magnetic heads to position themselves at a practically constant height over their associated disk. In order to achieve a correct read/write process, it is very important that the slider be arranged in the right attitude so the proper lift be generated and thus the proper height of the head over the disk surface, typically between approximately 5 and 50 nm, be obtained.
In industry the trend is to manufacture ever smaller hard disks, the latter being utilized even in extremely small-dimensioned devices, like photo camera and digital video camera apparatuses. As a consequence the dimensions of the sliders and those of the other component parts, that must obviously maintain and improve the high quality and reliability features, are extremely reduced.
European patent EP-1029219-B1 discloses an optoelectronic apparatus for the dimension and/or shape error checking of mechanical pieces with complex shape, for example supports with a plurality of wings (or “E-blocks”) for magnetic read/write heads of a hard disk. The apparatus includes, among other things, a system for locking and referring the piece to be checked and detecting devices including an optoelectronic system, for example of the known shadow casting type. The piece to be checked and the optoelectronic system can accomplish, amongst other things, relative rotation displacements about a longitudinal axis. The optoelectronic system includes a light emitter, for example in the infrared wave length, transversal to the longitudinal axis and directed towards a receiver, for example of the photosensitive CCD (“Charge Coupled Device”) type. The support for magnetic heads to be checked is positioned between emitter and receiver and intercepts a portion of light emitted by the emitter during small clockwise and counterclockwise rotation displacements (swingings) performed by the optoelectronic system. As described in EP-1029219-B1 it is possible to detect geometric and shape features of the piece, relating, for example, to the position of some parts with respect to a mechanical reference, by suitably processing the signal provided by the receiver. The amount of the variation, in the course of the rotation, of the portion of light intercepted by the part to be checked determines the “dynamics” of the signal provided by the receiver. As the dimensions, more specifically the transversal dimensions, of the part to be checked and of the swinging angle of the optoelectronic system decrease, the variations of the intercepted light and, as a consequence, the dynamics of the signal decrease. If the dimensions of the part to be checked are very small (for example less than a millimeter) and the swinging angle is limited, for example by physical features of the specific application, the limited dynamics of the processed signal may not guarantee the achieving of reliable results, even in consideration of unavoidable optical noises whose negative influence increases as the dynamics of the useful signal decreases.
The apparatus according to patent EP-1029219-B1 can be utilized for checking the arrangement and other features of sliders for hard disks, but if the transversal dimensions of the sliders are considerably small, there can occur the previously mentioned problems of scarce reliability.